Systems based on using multiple antennas are one of the most promising techniques for achieving high data rates. By combining MIMO (multiple input multiple output) techniques with OFDM (orthogonal frequency division multiplexing) modulation, the frequency selective MIMO channel is turned into a set of frequency flat MIMO fading channels which can be individually processed.
If a channel is known at a transmitter side, the channel matrix can be decomposed for each sub-carrier using SVD (singular value decomposition). As a result a set of orthogonal sub-channels is obtained in the space domain. These elementary sub-channels are also called eigenmodes.
One prior art method to find the optimal bit and power allocation for a set of multiple parallel sub-channels (for example, for eigenmodes of a MIMO-OFDM system) is Hughes-Hartog (HH) algorithm. Hughes-Hartog algorithm is depicted in John A. C: Bingham: Multicarrier Modulation for Data Transmission: An Idea Whose Time Has Come, IEEE Communications Magazine, pp. 5-14, May 1990, which is taken herein as a reference. However, the required computational effort of Hughes-Hartog algorithm increases with the average bit rate and therefore it is not a practical solution for high bit rate systems
Some sub-optimal fast loading algorithms have been proposed in Peter S. Chow, John M. Cioffi and John A. C. Bingham: A Practical Discrete Multitone Transceiver Loading Algorithm over Spectral Shaped Channels, IEEE Trans. on Communications, vol. 43, no. 2/3/4, pp. 773-775, February/March/April 1995, Robert F. H. Fischer and Johannes B. Huber: A New Loading Algorithm for Discrete Multitone Transmission, IEEE Proceeding of Global Telecommunications Conferences, vol. 1, pp. 724-728, November 1996 and Yu Wei and John M. Cioffi: On Constant Power Water-Filling”, IEEE Proceeding of International Conference on Communications, vol. 6, pp. 1665-1669, June 2001 which are taken herein as a reference.
These prior art methods are developed for slow varying channels like asymmetric digital subscriber line. For such channels, the signalling overhead required, for instance, for informing a receiver about the modulation scheme of each eigenmode can be neglected due to possibility to update the transmission parameters at relatively long time intervals. However, when the channel is changing fast, as is the case in wireless systems, the transmission parameters can be kept constant only for a time period shorter than the channel coherence time and therefore the amount of signalling overhead is significant.